The present invention relates to a method and apparatus for fluid compression of flowable plastic material following injection of the plastic into a mold cavity whereby to form a solid injection molded part having no internal voids, that is substantially strain fee, and has a Class A finish and sink-free surface.
Injection molds typically comprise stationary and moving mold halves (i.e., the core side and cavity side) which are closed and clamped together to form a mold cavity therebetween for shaping articles from thermoplastic compositions. The thermoplastic is heated into a molten condition and injected under pressure through a nozzle and into the mold cavity by means of a screw ram. Injection pressures of 2,000 to 10,000 psi are common at the gate locations. The plastic is allowed to cool to sufficiently harden the thermoplastic whereupon the mold is opened and the hardened articles removed.
A traditional plastic molding problem is the formation of surface distortions or xe2x80x9csink marksxe2x80x9d on the appearance side caused by ribs or bosses on the backside of a part resulting from the volume contracting (i.e., shrinkage) of the plastic during cooling. Further, warpage or part distortion can result from the high injection pressures used to fill the cavity, the pack out pressure, or from an uneven pressure gradient resulting from the injection pressure at the gate being higher than the pressures at the extreme ends of the molding. High injection pressures can cause strain marks or molded in strain in the hardened article, resulting in warpage at once, or over a period of time after molding, or if the end use of the molding is in a high temperature area. When ribs are formed in the molding, due to a shrinkage differential, the wall thickness versus rib configuration can cause the ribs to buckle or bend the molding. In large projected area moldings where the plastic cannot flow from the gate to the end of the molding, hot runner molds are needed and high clamping forces (e.g., 1,000 to 5,000 tons) are required to hold the mold halves together. These molds are costlier to build, and the runners can add a weld line to the product. Machines which can provide these high clamping forces are costly to operate.
In what has come to be known as xe2x80x9cgas assisted injection moldingxe2x80x9d an inert gas is injected through the plastic injection nozzle and directly into the thick areas of the melted thermoplastic, whereby to create hollow sections in the part. With the gas assisted molding process, sink marks and warpage can be minimized, and possibly eliminated. The gas is directed through a hollow (.e., gas channel) of the material formed between the surface of the part and a backside detail, such as a rib. If so, the base of the rib must be made thicker to help direct the gas channel, which is just the opposite of normal design practice with plastic where ribs are made as thin as possible to try and eliminate shrinkage. With the gas channel at the base of a rib, material will shrink away from the inside surface of the channel as the molded part cools because the material is the hottest at the center of the section. Therefore, as the plastic part shrinks during cooling, the sink mark on the visible outside surface is minimized.
A disadvantage in such gas assisted molding operations is that the gas pressure within the channels must be released prior to opening the mold, which normally requires costly post molding steps of venting the pressurized gas to atmosphere and then sealing or finishing this hole. Oftentimes sealing of this vent hole is needed, such as where the appearance or function of the part is affected, or to obviate the possibility of the part contaminating various chemical baths during secondary operations, such as chrome plating or painting.
Additionally, the possibility of achieving a Class A surface is inhibited by shadow marks caused by gas holes in the thicker areas of the molding, and gas permeation caused by the gas not being retained in the thicker areas and overflowing into the wall thickness of the molding. This causes thinning and weakening of the wall, raised areas, and blush marks.
In the gas assisted process, the gas used during the molding operation can be recovered to some extent but the chances are it will be full of volatiles from the molded polymer which would need to be removed. However, there are dangers in compressing inert gas with a volatile gas (e.g., fire).
Additionally, with gas assistance, costly apparatus is needed in the form of gas compression units, nozzles, pins and the like to introduce the gas into the molding. Further, to operate these units at the high pressures needed (e.g., 9,000 psi) is energy costly, the gas used and lost is costly, and the cost of maintenance is high.
Injection molding of parts utilizing a pressurized gas source is shown in xe2x80x9cInjection Mold Method and Apparatus,xe2x80x9d published Jun. 14, 1990 as PCT Publication WO 90/06220, the specification being specifically incorporated herein by reference. While this process is suitable for molding articles of the type shown therein, there is always a need for improvements in forming low cost articles.
The primary objects of this invention are to provide a method and apparatus which enhances the low cost production of a plastic molded part which is stress-free, has a Class A surface condition, is free of xe2x80x9csink marksxe2x80x9d or xe2x80x9cblush marksxe2x80x9d, and has no gas internally in the part or voids internally of the plastic, avoids permeation and witness lines, does not require venting the fluid pressure within the molded part, provides a constant gas pressure across an inner surface of molten plastic used to form the mold part, and allows for the reclaiming of the fluid (i.e., gas) with reduced volatile content for reuse in the process.
A further object of this invention is provision of a self-sealing arrangement during molding and curing to prevent air in the mold cavity or the forming gas from either migrating around the thermoplastic. Undesirably, this gas could force the molten plastic away from the mold cavity surface used to form the finished surface or escape across the parting line of mold sections and pass outwardly from the mold cavity.
According to the self-sealing aspect of this invention, a continuous groove (or recess) is formed in one of the mold cavity surfaces which cooperates to form a gas sealing ring during the introduction of thermoplastic material into the mold cavity, prior to compression molding the thermoplastic.
Advantageously, the gas sealing ring thus formed inhibits unwanted migration of air to the appearance surface.
However, in some applications, if the sealing recess is not substantially completely filled, it is possible that air could be trapped in the sealing recess. If so, a xe2x80x9chalf moonxe2x80x9d can be formed in one corner of the thermoplastic, resulting in entry of air which will push the thermoplastic material away from the sealing recess and thereby allow the air to migrate to the other side of the thermoplastic material.
Accordingly, still another object of this invention is the provision of an arrangement for sealing and evacuating air from the mold cavity and in the gas sealing recess formed in a surface of the mold cavity to less than atmospheric (i.e., a negative pressure) prior to the introduction of a mass of thermoplastic into the mold cavity whereby to ensure that air is not trapped in the gas sealing recess during introduction of the thermoplastic, which trapped air could be forced to the appearance surface of the molded part during introduction of compressed gas.
Another object of this invention is the provision of arrangements for evacuating air in a mold cavity directly from the gas sealing recess.
Still another object of this invention is provision of an arrangement for evacuating air in a mold cavity and a gas sealing recess therein directly via a bleed line adjacent to the gas introduction inlet.
Yet another object of this invention is provision of mold apparatus which eliminates the need for gas channels to communicate gas to remote locations whereby to form free-standing bosses, stiffeners, and other structural details.
A further object of this invention is provision of an injection molded, gas compressed, dimensionally stable, thermoplastic part having reduced wall thicknesses, without the need for either reinforcement ribs, as desired, or internal gas cavities.
Yet another object of this invention is provision of a process that is efficient, requires lesser pressure to form a part, reduces the clamping forces needed to retain the molds together against the pressure, obviates venting, and advantageously uses at least part of the forming pressure to assist in ejection of the finished part upon opening of the mold portions.
Yet a further object of this invention is provision of fluid inlets which are efficient to provide a uniform gas pressure across the inner surface of the injected thermoplastic and are less costly than conventional nozzles and injection valves.
A further object of this invention is provision of a gas recirculation arrangement that enhances the cooling of the part.
A method of and apparatus for fluid compression of injection molded plastic material are provided to form a strain free part having no internal voids, a Class A finish, and sink-free surface. The apparatus comprises stationary and movable mold portions which has part forming surfaces and are moved from an open to a closed position to define a mold cavity of the shape of the desired part, at least one injection valve for introducing melted thermoplastic into the mold cavity, and at least one gas inlet valve for introducing pressurized gas into the cavity. A continuous sealing groove (or recess) is formed in one of the part forming surfaces and in encircling relation to the gas inlets to receive and form a thermoplastic seal ring which prevents gas from escaping from the mold cavity across the parting line and/or from migrating around the thermoplastic to the appearance side of the thermoplastic.
According to the method of producing a compression injection molded part having no internal voids and a Class A finish, thermoplastic material is introduced into the mold cavity and the sealing groove, a cushion of gas is introduced into the mold cavity at a selected location to apply an external pressure on one side of the thermoplastic material which moves the thermoplastic material at that location inwardly of the mold cavity and away from the adjacent mold cavity surface, and maintaining the external pressure until the thermoplastic is self supporting.
According to this invention, prior to the step of introducing the thermoplastic material into the sealing groove, the air in the sealing groove is reduced to a pressure below atmospheric whereby the sealing groove substantially entirely fills with thermoplastic without trapping air therewithin. In one embodiment, the sealing groove is evacuated via a bleed line extending from the mold cavity and in part between the parting plane of the mold portions, the parting plane otherwise being clamped in airtight relation. In another embodiment, the sealing groove is evacuated by a bleed line opening directly onto a wall thereof. In another embodiment, the sealing groove is evacuated in part by a configured sealing pin clearance fit in a shaped bore opening into the groove.
The pressurized gas, such as nitrogen, operates to uniformly force the molten thermoplastic away from one mold portion and against the other mold portion to form the finished outer surface of the part.
Advantageously, evacuating the sealing groove prior to introducing the molten thermoplastic will ensure that the thermoplastic will not trap any air in the sealing groove which could otherwise prevent the sealing groove from filling completely or cause weak spots to form in the gas seal ring. This can be important because even if the mold cavity is vented to let the air which might otherwise escape to the other side of the seal ring, the air pressure in the mold cavity will increase inasmuch as the air cannot evacuate the mold cavity fast enough to obviate an increase in air pressure, which pressure could stop the sealing groove from completely filling.
Preferably, and according to this invention, the stationary portion (i.e., the core side) can include a plurality of recesses to form free-standing bosses, or an upwardly extending core body having a plurality of rib forming chambers that fill with thermoplastic. The chambers form ribs on the inner side of the part and these ribs can be continuous, interrupted, and used in combination with the recesses to form free-standing bosses. Additionally, the walls of the rib forming chambers can be provided with a stepped portion to form an increased volume area for the plastic to flow into to drive the plastic outwardly whereby to inhibit shrinkage of the part and separation of its exterior appearance surface from contact with the mold cavity surface.
Advantageously, the method and apparatus herein allows formation of free-standing bosses and ribs without the need for gas channels which extend thereto, as is needed with traditional gas assisted injection molding. This allows flexibility in design whereby all ribs that are not needed for strengthening the molding, but are only there to inhibit shrinkage in remote areas of thick sections of the mold, can be eliminated.
Further, moldings can be made using low pressures and low clamping forces, and eliminating the need for hot runners.
Advantageously, the pressurized gas uniformly urges the plastic away from one of the mold portions and against the other mold portion to inhibit shrinkage of the molded part from contact with the mold surface of the other mold portion, and can also be used to eject the part from the mold cavity, whereby to obviate the use of ejector pins which can cause surface indentations.
Advantageously, control over the gas utilized can be reclaimed to save energy costs and gas costs, and recirculated to enhance cooling and reduce time required for the molds to cool between cycles.
A further advantage of a mold apparatus having the rib forming channels is provision of a reinforced structural part while providing a surface having a sink-free Class A finish free of permeation marks, blush marks, etc.
Additionally, gas compression of injection molded plastic allows formation of structural parts of thin and thick cross-sections.